1. Field of the Invention
This invention relates generally to processes for producing and utilizing small diameter jets of high pressure fluid with entrained abrasive particles as abrasive tools. This invention relates more specifically to processes for the forming and use of small diameter coherent abrasive suspension jets of high pressure fluid with entrained abrasive particles directed to the cutting of materials.
2. Description of Related Art
Jets of high pressure fluid have been used for over 100 years in mining to wash ore bearing gravel from cliffs and stream banks. More recently, using pressures up to 55,000 psi, water jets have been used to cut materials that are ordinarily cut with knives, shears or saws. The entrainment of abrasive particles in these water jets has permitted cutting of hard materials such as steel, concrete, and lightweight composites.
In a typical water/abrasive jet system, abrasive particles are entrained in the water jet after the jet is formed by an orifice. The water jet and entrained particles are then collimated by a director or focusing tube and are allowed to impinge on a target. In the entrainment process, mixing inefficiencies prevent the abrasive particles from being accelerated to jet velocity. The velocity of the final jet is further inhibited by the director tube, because the director tube typically has a larger diameter than the orifice which forms the jet, resulting in jet dispersion and a reduction of the jet velocity.
There are, therefore, two primary disadvantages to the method of entraining abrasive particles after the jet is formed. First, the jet leaves the abrasive mixing head at a velocity significantly below the initial velocity of the primary jet. Second, the mixing requires that there be some dispersion of the initial primary jet and the process of recollimating and focusing the mixed flow necessarily fails to achieve the narrower cross section of the primary jet orifice. These two disadvantages will be discussed in more detail below.
There are a number of derivative disadvantages to the method of entraining the abrasive after the jet is formed. Ordinarily, abrasive water jet cutting, in which a dry abrasive is fed into a mixing chamber and combined with the water jet, produces sparking, especially on the back side of a metal target when the jet has struck through. This sparking has been sufficient to discourage the use of abrasive jet cutting in hazardous atmospheres. It has been found, however, that no sparking occurs when the abrasive is introduced into the mixing chamber of the jet head already in a fluid mixture. Apparently, dry abrasives are not fully wetted by the jet and can strike sparks, while wetted abrasives transfer sufficient energy to the absorbed water to prevent sparking. It has also been found that feeding dry abrasive in a conventional jet leads to static build up in the feed line, and sometimes leads to static discharge.
An additional disadvantage to a conventional water/abrasive jet is that it can not be used under water without much difficulty. Two factors create this difficulty. First, the abrasive feed must be pressurized to prevent ambient water from rising into the mixing chamber, and second, the secondary jet with entrained abrasive tends to break up while traversing ambient water. For these reasons, the jet must work very close to the surface being cut.
The structure of a conventional water/abrasive jet nozzle is shown in FIG. 1. High pressure water flow 1 enters the conventional jet head by way of inlet tube 2. Dry abrasive flow 7 enters the conventional jet head by way of feed hose 8. High pressure water flow 1 is forced through orifice 3 and results in primary water jet 4. Orifice 3 is typically made of sapphire (or other hard material) and is on the order of 0.01-0.05 inches in diameter. Primary water jet 4 combines with dry abrasive flow 7 in chamber 4a where it is focused by tungsten carbide focusing cone 5, and further collimated by tungsten carbide focusing tube 6. This results in a collimated jet 9 of water and aspirated abrasive. A sapphire orifice is used to create the high velocity, primary jet of water, because of its ability to withstand wear. Typical pressures for water flow in the primary jet range from 14,000 to 55,000 psi. The abrasive, which is usually garnet sand, is aspirated into the mixing chamber by the action of the jet, mixed with the jet, and the two are reformed into a secondary, lower velocity jet by means of the focusing cone and focusing tube.
The velocity of the collimated, secondary jet may be increased by increasing the water flow to the primary jet. This increased water flow may be achieved by using a larger diameter orifice, a higher driving pressure, or both. The use of smaller diameter focusing tube may also be used to increase the velocity of the secondary jet If the water pressure is made to increase, then depending upon the primary jet flow and the focusing tube diameter, water may enter the abrasive feed line and stop the abrasive flow. Accurate alignment of the focusing tube with a center line of the primary jet is required in order to obtain a well collimated secondary jet and to decrease tube wear. Inefficiencies created by the momentum exchange between the primary jet and the abrasive particles reduce the cutting efficiency of the collimated jet. There are, therefore, certain inherent limits to the primary jet pressure and to the focusing tube diameter (and thus the secondary jet velocity) that prevent such mixing head devices from overcoming the primary disadvantages mentioned above.
The overall complexity of such mixing head type abrasive jet systems can itself become a problem. While the primary jet orifice can be very small and compact, the mixing head assembly requires in addition to the orifice a mixing chamber, collimating cones and tubes, and most importantly, two feed lines. Besides the problems associated with increased nozzle size, the necessity of a second supply line that is capable of transporting abrasive particles can often mean the difference between a practical application and one that is impractical.
Apart from the inherent complexities of a mixing type jet head, the process of entraining particles in the jet stream after its formation also implies a more complex supply system. While providing high pressure water as a working fluid at a mixing head is relatively simple, the supply of dry or slurried abrasive particles can be anything but straight forward. Dry abrasives most often are conducted by a gas stream, which must not only be produced and maintained, but must be dealt with at the point of mixing with the primary jet stream. Slurried abrasive streams must generally be agitated in order to prevent settling and to maintain a proper flow through to the mixing head. More recent attempts at conducting abrasive particles in a foam medium have improved abrasive flow but have not reduced the complexity required by the second supply line to the mixing head.
A typical conventional water/abrasive jet, as described above, operating at a pressure of 30,000 psi, with an orifice diameter of 0.01 inch, will cut 0.25 inch thick steel with a traverse speed of approximately 4" per minute, a jet power of approximately 6.15 hp, and a resultant jet work per inch cut of approximately 1.53 hp-minute per inch. Such conventional jets typically consume abrasives on the order of 0.6 lbs. per minute with a resultant 0.15 lbs. per inch abrasive consumption. Typical water use for a conventional jet is 20 cubic inches of water per inch of cut.
If the mixing of the abrasive and working fluid could be accomplished prior to the formation of the primary jet at the orifice, then both the fluid and the abrasive could be expelled from the orifice at the same velocity. In such a system, a focusing tube would no longer be required and the abrasive jet would impinge directly on the target. Cutting efficiency would be increased because of the higher abrasive particle velocities, and the narrower jet cross section.
The inability to accomplish this premixing of the abrasive and working fluid has resulted primarily from an inability to maintain the abrasive in a suspended, transportable state within the fluid. Various methods of forming slurries and/or foams with abrasives have overcome some of the problems associated with the pumping and transport of the abrasive to the jet nozzle, as described above, but none of these processes have achieved the capability of transporting a fully mixed working fluid through jet orifices smaller than 0.020" in diameter. Most such previous attempts have continued to rely on the more complex structure of a mixing type nozzle, wherein a high pressure water flow is made to combine with a lower pressure slurry or foam abrasive mixture flow. While certain disadvantages described above are overcome with slurry and foam mixture flows, ultimately the two problems which most severely effect the function of an abrasive jet, namely loss of velocity and loss of coherence, remain problems because of continued requirement of post jet mixing and secondary collimation.